SUMMARY - Draper 121002 MS-3R & MS-4R 2-station control:

The MS-3R and MS-4R are 3- and 4-wire versions of the same switch,
used for controlling one screen from multiple locations, using the
motorís voltage (115 or 220).

MS-4R is the first control station in a series of stations. It is also
the intermediate control in the case of three or more stations. The
fourth wire passes electricity to the next switch in the series. This
fourth wire is taped off to create the MS-3R. MS-3R is the last
control station in a series of stations. Therefore, it does not need
to
pass electricity to a next switch. It is used with one or more MS-4R
controls.